In order to produce articles in which a sheet-shaped label is bonded to the surface of a molded body made of a synthetic resin, a method has been widely and generally used, which comprises forming molded body by injection molding or blow molding, opening a molding mold to remove the molded body, and manually bonding adhesive-applied labels to outer peripheral surfaces of thus obtained molded bodies one by one or automatically bonding the labels to them by a label-bonding machine.
However, the former manually label-bonding method has the problems in that the manually label-bonding makes the workability worse, and is likely to cause variations in the bonding precision depending upon working persons. As compared with the former manual working, the latter automatically bonding method with the bonding machine has improved workability and the bonding precision can be enhanced by diminishing variations. However, the latter method is disadvantageous from the standpoint of the cost in that the machine is of a large scale, and needs a large machine cost. Further, since the labels are bonded in both of these bonding methods after the synthetic resin-molded bodies are molded, the bonding work itself unfavorably becomes troublesome.
Under these circumferences, JP-A 63-242,613 and JP-A 4-14,420 disclose methods in which label bonding is effected simultaneously with molding the synthetic resin during the molding step, while the troublesome work of forming the above molded body and then bonding label to the surface of the molded body is omitted. According to these publications, a so-called insert molding is effected, which preliminarily placing a label or the like in a molding mold, blow-molding or injection molding a resin and integrating the molded body and the label or the like.
However, although no great problem occurs in the production of the insert molded body by the blow molding as in the former JP-A 63-242,613, close attention is needed when the insert molded body is produced by the injection molding as in the latter JP-A 4-14,420, because the molten resin oozes out to the surface side of the label, and the label is wrinkled.
When an insert molded body is to be produced by injection molding, an insert 50 such as a label or the like is inserted into a predetermined position of an inner surface 51 of an outer mold 51 as shown in FIG. 8(a) in the state that a molding mold units 51 and 52 are opened, the label is fixed by suction or the like, and injecting the molten resin into a molding cavity of a mold by an injection molding machine 53 after the molding mold units 51 and 52 are closed as shown in FIG. 8(b).
When the molten resin is to be injected into the molding space inside the molding mold, the molten resin is generally injected into the molding cavity through a gate portion 54 provided at an end portion of the molding space. In this case, the molten resin may flow to the surface of the label owing to various factors such as the label 50 being not closely attached the shape of the end portion of the label, the shape of the end portions of the label, the relationship among the flow, the pressure, etc. of the resin.
FIG. 9 is a sectional view of an apparatus for producing a conventional label-bonded cylindrical molded article, which is used for effecting the insert label molding for a cylindrical molded body by injection molding. In the figure, 61 is a molding mold end disc, 62 a mouth portion-molding mold unit for forming a mouth portion of the cylindrical molded body, 63 a barrel portion-molding mold unit, 66 a core, 67 a molding cavity, 67 a mold cavity, 68 a knock-out pin, and 69 an injection nozzle. As shown in FIG. 9, a molten resin is injected through the injection nozzle 69 and an upper end portion of the molding space, it is likely that the molten resin enters between the surface of the label 60 and the barrel portion-molding mold unit 63 and the molten resin pushes down the label 60. Consequently, unacceptable products were often produced.
As countermeasures for removing causes resulting in occurrence of such unacceptable products, the insert is prevented from being deformed or moved by imparting a force resisting the flow of the molten resin through increasing the thickness of the insert such as label or by firmly closely attaching the insert to the inner surface of the molding cavity with the provision of a vacuum sucking means in the molding mold as shown in FIGS. 8(a) and 8(b).
However, if the insert is made thicker, the cost of the material becomes higher. On the other hand, when the molding mold is provided with the vacuum sucking means to suck the insert 50, 60 such as the label to the inner surface of the molding cavity, the molding apparatus becomes complicated. Therefore, each of these cases results in the increased cost of the products.
In order to simplify the structure of the molding mold to be used for the above insert molding, outer molds of a split mold structure are generally used. However, if molding is effected with such a molding mold, a parting line of the molding mold can not be prevented from being formed at the surface of the molded body.